1. Field of the Invention
This invention relates to a method of detecting and monitoring damage in a structure by monitoring acoustic energy transmitted within the structure. The invention relates, in addition, to damage detection and monitoring apparatus and to a structure including a plurality of acoustic transducers.
2. Discussion of Prior Art
It is known that stable crack growth in metallic structures due to fatigue and stress corrosion occurs due to a process of slow embrittlement of the material as a result of stress concentration within a short distance of the crack tip. This is followed by crack advance to a zone boundary in a series of discrete transgranular or intergranular microfracture events, the advance being arrested at the zone boundary by tough undegraded material. This cycle then repeats. The microfracture events are explosive in nature, those occurring in steel and aluminium alloys typically occurring with a mean velocity in the range of from 250 to 500 meters per second.
Structures in which fracturing of this kind is a particular problem include offshore oil and gas installations, aircraft, and pressure vessels. The invention has primary application to metallic structures in those technical fields, but it is not limited by the field of application nor by the material used.
The applicants recognise that crack development, as described above, produces wideband ultrasonic energy (acoustic emissions) within the structure which can be detected using transducers. The range of detection of acoustic pulses depends on such factors as the material thickness, component shape, surrounding fluid medium and interference from background noise.
It is an object of this invention to provide a method aiding the detection and/or location of damage sites of a range of sizes.
According to a first aspect of this invention, a method of detecting and monitoring damage in a structure comprises: receiving continuously over a period of time electrical signals from a plurality of acoustic transducers carried by the structure; and in a pulse processor,
(a) forming digital representations pulses from the electrical signals to form data bursts,
(b) selecting data bursts that occur in a predetermined time window to form a group of data bursts,
(c) deriving, for each of the selected data bursts, delta-t values representing the differences between the times of occurrence of the pulses represented by the selected data bursts in the groups, the delta-t values forming a delta-t pattern, and
(d) generating a significant event indication signal when the delta-t pattern is repeated to a predetermined degree in different groups of selected data bursts.
According to a second aspect of the invention, apparatus for use in the detection and monitoring of damage in a structure comprises: a plurality of acoustic transducers for mounting on a structure to be monitored; a signal conditioning stage coupled to the transducers and arranged to derive pulses corresponding to acoustic events sensed by the transducers; and a pulse processor unit comprising an analogue-to-digital converter (ADC) stage coupled to the signal conditioning stage and a digital signal processing stage including (a) digitising means to digitise pulses from received signals to form data bursts, b)selection means configured to select data bursts to form a group of data bursts emanating from the signals received from different transducers in a respective time window, (c) means arranged to derive, for each of the selected data bursts, delta-t values representing the differences between the times of occurrence of the data bursts in the group, the delta-t values forming a delta-t pattern, and (d) a correlator arranged to generate a significant event indication signal when the delta-t pattern is repeated to a predetermined degree in different groups.
The invention also includes a structure which is monitored for damage due to cyclic loading, wherein the structure comprises a plurality of structural members, a plurality of groups of acoustic transducers, each group mounted on a respective structural member, and a signal processing unit coupled to the transducers and arranged to detect groups of data bursts each comprising a data burst representing pulses in signals received from respective transducers of one of the groups of transducers during a sampling time interval, and to derive, for each of the data bursts, timing information relating to the times of arrival of the pulses forming the group, the signal processing unit further comprising a correlator arranged to generate a significant event indication signal when the timing pattern is repeated to a predetermined degree in different groups from the said group of transducers.
The processing unit is configured to have a plurality of channels, each channel handling signals from a respective transducer.
Each group of data bursts is typically composed of data bursts in different respective channels of the unit occurring within a respective time window, continuous data acquisition occurring over long time periods, typically in the order of hours to several months, during which bursts are received continuously from consecutive or overlapping such time windows. Depending on the nature of the structure and the loading under which it is placed, relevant significant acoustic events due, for instance, to growth of a crack may occur only once in millions of data burst windows, typical causes being extreme weather conditions in the case of an oil rig, or certain high load events during an aircraft""s flight (for instance takeoff and landing, or turbulence).
Damage, including fractures or cracks in metallic structures and fretting in composite structures invariably give rise to these significant acoustic events which the method and apparatus according to the invention aim to detect and to indicate as a so-called significant event indication signal.
The ability to detect and monitor crack growth sites of a wide range of sizes and different acoustic emission intensities is enhanced if groups of data bursts are correlated using real time filtering via a primary filter with a first set of characteristics selected to detect new fractures, and secondary filters which are adaptively created with a second set of characteristics to monitor fractures which have already been detected using the primary filter. The secondary filter characteristics are tailored to the characteristics of groups of data bursts emanating from these detected fractures. Such groups are then diverted from the primary filter to be processed instead in the secondary filter. This allows, for instance, high and low intensity damage sites to be simultaneously monitored without substantial loss of data due to such causes as lack of processing capacity or storage device overflow.
Other preferred features are contained in the dependent claims accompanying this description.